1. Technical Field
The present invention relates to the rear axlecase of a vehicle, and particularly to those rear axlecases which are made of a nodular graphite cast iron casting, in which the endtubes of the axle housing are improved in terms of material quality.
2. Background Art
The conventional construction of the automobile rear axlecase that houses a differential gear and the two axles that extend laterally from the differential gear can be described as follows:
As shown in FIG. 7 of the accompanying drawings, which presents an example of conventional design, the rear axlecase 1 is made up mainly of a differential gear housing 2 and two axle housings 3, which extend laterally in opposite directions from the differential gear housing 2. At either end of the axle housings 3, there is formed an endtube 4, and at the differential gear housing 2 side thereof, there is formed a brake flange 5. In the top part of the differential gear housing 2, there is formed one or more pieces of a bracket 6. At the top part of each axle housing 3, there is formed a spring seat 7, while at a bottom part thereof, there is formed one or more pieces of torquerod bracket 8. Here, the endtubes 4 are made of a casting of good mechanical properties, while most of the other parts are press formed, and these members are assembled as by welding into a complete rear axlecase 1.
In this example, because plural members are assembled by welding, not only is productivity low, but the weldments are liable to become weak in structural integrity. One further problem here is that, should a weldment be found deficient in strength, the only remedial measure possible would be redesigning to enlarge the fundamental dimensions, thereby necessitating alteration of the form of the parts. Another problem is that, since press dies are expensive to make, this method is not suitable to flexible manufacturing.
In another example, attempts at overcoming these difficulties were made by resorting to a construction, in which the above mentioned plural members were all cast into one integral body from a cast steel, or alternatively, the differential gear housing 2 and the axle housing 3 were cast into one integral body from cast steel, and brackets 6 and the like were attached thereto by means of bolts or welding, so as to be assembled into a complete rear axlecase 1.
The problem with this latter approach was that the castability of cast steels is poor due to the need for high temperatures and the formation of shrinkage cavities; the requirements imposed on the casting mold are severe thereby raising production costs, and manufacture of thin wall articles is difficult, making weight reduction difficult.
To solve these problems, yet another construction has been devised. In this third conventional practice, an example of which is shown in FIG. 8, the endtubes 4 of the axle housing 3, which require a high degree of toughness, are made from a foreging, which is known to have good mechanical properties, while the parts of the axle housing 3 other than the endtubes 4, whose deficiency in strength may be corrected by adjusting the form and dimensions, are cast into an integral body together with the differential gear housing 2 from nodular graphite cast iron, and forgings and nodular graphite cast iron castings are assembled with bolts.
The problem here is that, since welding of forgings to nodular graphite cast iron casting is difficult, if not entirely impossible using technology available today, and since, even if successfully welded, the strength of weldment is insufficient, the forging part and casting part have to be joined by means of bolt, thereby necessarily raising the production cost.